The present invention relates to an exhaust emission control system of a combustion engine, particularly to an engine control system for exhaust purification of a lean-burn engine combustable at a wide air-fuel ratio.
The lean-burn engine has attracted its attention as the needs for fuel-efficient engines increase. The lean-burn engine is generally equipped with a NOx trap catalyst in the exhaust pipe for purifying NOx during lean operation. The NOx trap catalyst has the following functions, that is, a function which traps NOx in an oxidation atmosphere (at the time of lean operating), and a function which releases and reduces NOx in a reduction atmosphere by HC and CO contained in exhaust emission from the engine (at the time of rich operating).
Accordingly, in order to decrease NOx in the exhaust, it is important to utilize the NOx catalyst efficiently, and to optimize both the timing for changing to a reduction atmosphere (the timing for starting a rich spike) and the amount of reduction agents (rich spike amount) to be supplied. According to the prior arts, the following inventions are proposed. For example in Japanese Application Patent Laid-Open Publication No. 2001-271679, a NOx sensor is provided in the downstream of the NOx catalyst to detect the termination time of the rich spike.
In Japanese Application Patent Laid-Open Publication No. Hei 11-229853, and Japanese Application Patent Laid-Open Publication No. 2000-337131, a NOx sensor is provided in the downstream of the NOx catalyst to diagnose degradation of the NOx catalyst.